Anemometers are instruments that are used to measure fluid velocity, such as the velocity of a gas or gasses, e.g., in a duct. Anemometers are used most commonly to measure airflow in a wide variety of scientific and industrial applications. In addition to measuring airflow, anemometers can also be configured to measure air pressure. This is because there is a close connection between the pressure and speed of flowing air. Airflow measurement remains the primary use for anemometers.
Anemometers can have a variety of configurations. Hot-point anemometers operate on the principle that air flowing past a heated structure has a cooling effect on that structure, and this cooling effect is proportional to the velocity of the air. In a hot-point anemometer, a hot-point element is a temperature sensitive element, such as a resistive element configured in a circuit (e.g., a Wheatstone bridge circuit) that heats the element electrically, typically with either a constant current or a constant voltage. The hot-point element is physically exposed to the airstream in which airflow is being measured. Air flowing past the element will have a cooling effect on the element, which changes the electrical resistance of the element. These changes in temperature/resistance are proportional to the velocity of the measured airflow and are reflected in the voltage output of the anemometer circuit.
Hot-point anemometers are typically configured to include a probe that is inserted into the airstream in which the airflow is to be measured. The probe, which can have a typical elongated tube configuration, can include an aperture .or opening through which air in the stream can pass. The hot-point element is exposed in this aperture. The hot-point element can be any of a variety of types.
One type of hot-point anemometer utilizes a very fine wire as the hot-point element. Commonly referred to as hot-wire anemometers, the wire used in these devices is typically a very small gauge metal wire (e.g., on the order of several micrometers), typically tungsten, that has a relatively high resistive response to temperature changes.
Another type of hot-point anemometer incorporates the use of a thermistor, such as bead thermistor, as the hot-point element. Thermistors are a class⋅ of resistors whose resistance varies significantly with temperature and, as such, are well suited to for implementation in devices, such as anemometers, that measure temperature as a function of electrical resistance.
Another type of hot-point anemometer is a hot-film anemometer, which implements as the hot-point element a thin metallic film deposited on a semiconductor (e.g., Si or SiN) substrate. The film can have a structured configuration or pattern selected to provide a desired degree of exposure to the airstream. Hot-film elements can be advantageous because the sensitive sensor component can be distributed and patterned on a surface rather than being constrained, for example, by a wire or bead configuration. This flexibility in sensor configuration can, for example, be used to provide the hot-film element with a quick response time due to low heat capacity and low heat conduction due to a distributed sensor pattern, provided the substrate has a relatively low heat capacity.
Anemometers can implement electrical circuits that are configured in a variety of manners to measure airflow via the hot-point element. For example, a constant current anemometer (“CCA”) implements. an electrical circuit that applies a constant current to the hot-point element. The voltage output from the circuit is the result of the circuit's maintaining the constant current given the change in resistance of the hot-point elements resulting from airflow. Similarly, a constant voltage anemometer (“CV A”) implements an electrical circuit that applies a constant voltage to the hot-point element. The voltage output from the circuit is the result of the circuit's maintaining the constant voltage given the change in resistance of the hot-point elements resulting from airflow.
Additionally, a constant temperature anemometer (“CTA”) implements an electrical circuit that is configured to maintain the hot-point element at a constant temperature. The voltage output from the circuit is the result of the circuit's maintaining the constant temperature of the hot-point element despite the cooling/heating effects of airflow. Further, pulse-width modulation (“PWM”) anemometers are configured to infer the air velocity from the amount of time required for the wire to reach a specified resistance while a pulsed current of a predetermined frequency, duration, and amplitude is applied to the wire.
Hot-point anemometers are frequently used to measure airflow in environments where the airstream contains particulate contaminants such as dust, sand or stone. These contaminants can strike the hot-point element, which can cause damage the element. Over time, this damage can cause the anemometer to malfunction, lose accuracy, and ultimately limits its useful life.
These issues can be heightened with hot-wire anemometers. Since typical hot-wire is of a very small gauge, it is delicate and can be broken easily with significant impact. Since assembling hot-wire anemometers is typically cumbersome and done by hand, replacing broken hot-wire elements can be expensive. Additionally, anemometers that implement thermistors as hot-point elements make use of small bead thermistors, which can be similarly fragile when exposed in the air stream. Bead thermistor components typically include wire leads that require hand assembly.
Airflow measuring devices and instruments, commonly and collectively referred to as “airflow meters,” have widespread use in science and industry. All of these uses require certain levels of accuracy, durability, and reliability. For instance, in the building automation industry, it is important that airflow meters are robust and low-cost to maintain. Depending on the environment in which the airflow meter is used, this can be a challenge. Industrial applications can expose the meter to a variety of particulates in the airstream. Where the airflow meter is a hot-point anemometer, the risk of damage in these environments can be high. Therefore, there is a need for a hot-point anemometer for use in these environments that is both reliable and easy to manufacture.